1. Field of the Invention
Embodiments of the present invention relate to inspection systems, and more particularly, to automated x-ray inspection systems and corresponding methods.
2. Description of the Related Art
It is well known fact that during the assembly of some products, it is necessary to inspect the product at a stage in the assembly when the elements to be inspected are hidden from the human eye or a machine vision inspection system. Currently, the reduced size of one or more electronics components and the consequent increased density of items on printed circuit boards (PCBs) have caused the traditional ways of inspection to be no longer sufficient for an affordable and precise quality control inspection. Moreover, printed circuit boards are now produced in large quantities, and as they are expensive and are used in expensive equipment, it is important that they be produced accurately, with minimum wastage. Unfortunately, because of the manufacturing methods available, wastage because of rejects is still higher than in other industries. Furthermore, where a Direct Chip Attached (DCA) technology is used, in which the solder connections are hidden from view, fast accurate quality control inspection of the electronic devices is very difficult.
Currently, the V810's system architecture doesn't have the capability to measure the actual board height. Although existing autofocus including profile analyzer technique can estimate the correct solder slice for most of the joint types, it fails in cases where the joint for the inspection has large and thick neighboring components for e.g. tall capacitor, multilayer on one side and also when the joint or pad is thin.
Currently in the art, the PCB is scanned with a camera utilizing telecentric lenses, which views in parallel beam imaging to photograph a section of the board, and use software to analyze the image and in other cases the PCB is scanned with a camera utilizing fixed-focus lens. However, there remains a problem with this arrangement in which imaging camera is only focusing sharply within certain depth of field (DOF). Moreover, one or more objects that fall out of this range are blur and is out of machine's inspectability. Furthermore, there is a disadvantage with this arrangement in that the camera can only view the image in two directions, which is not a problem as long as the board is exactly flat, but although normally these boards are nominally flat, exact flatness is difficult, if not impossible to achieve for various reasons. Steps are taken to keep the board flat to the degree necessary to make the method accurate, but this is not easy, and in fact the board is sometimes clamped in an effort to keep it sufficiently flat to make the results acceptable. It should be borne in mind at this time that the dimensions of the degree of out of flatness (i.e. warpage) of the board which are involved are measured in microns, but even warpage of the board to this minute degree can have a serious effect on the accuracy of the inspection.
In one or more cases, penetrating radiation is employed to inspect products by producing images of the internal structure of the devices and the connections. Particularly, the solder joints that are located under the body of a component would not normally be visible but can be observed via X-ray inspection. Currently, most of the existing inspection systems use X-ray radiations and radiographic techniques. For example, the solder joints attaching some high density integrated circuits to a circuit board may be inspected only after the integrated circuits are in place covering the joints to be inspected. In mode of operation, an X-ray inspection system may be employed, which may include an X-ray source, opposing a detector about the part to be inspected. The X-ray source is normally an electron tube accelerating electrons in vacuum from a cathode to a fixed anode as focused by one or more grids. The X-ray radiation penetrates any obscuring structure to produce a shadow image or radiograph of the part from which the desired elements may be discerned as variations in X-ray attenuation. For example, a metallic solder joint may be detected as a region of high attenuation contrasting with the uniform and frequently lesser attenuation of the obscuring integrated circuit package and die.
Additionally, a rigid mounting of the X-ray source may be employed to reduce any physical shock to the X-ray source which may adversely affect the size of the X-ray source's focal spot. Thermal variations are reduced by leaving the X-ray source on at all times. The small focal spot of the X-ray source (less than one-hundredth the diameter of a typical medical X-ray source) is necessary to eliminate a penumbra that may blur a highly magnified image and is accomplished by multiple grids and a special X-ray source anode geometry.
However, while employing automated X-ray inspection (AXI) knowledge of the surface where solder joint are mounted to the printed circuit board (PCB) is important for classification. Inspection machine's call rate and accuracy is very much dependent on it. This information can be obtained either directly by surface height profile measurement or indirectly by calculating the image sharpness matrixes. However, both approaches are not perfect. Moreover, the direct surface height profile mapping cannot measure joints that are hidden from sight while the indirect software approach is not reliable for certain joint types. Furthermore, the current available AXI machine, implement the indirect approach.
Moreover, 3-D scanning laser mapping is slow due to the time required to obtain and process the 3D laser scan data. Furthermore, in 3-D scanning laser mapping in Agilent's 5DX machine (AXI) system height information is collected point by point and in other scenarios utilize line scanning. Consequently, the scanning speed became speed bottleneck to the 5DX system. In addition, setup time for laser mapping is also tedious with poor repeatability. The focused laser spot used in the measurement is bright and it tends to saturate the camera easily when the spot fall on copper traces (PCB) or high reflective component leads. Moreover, this information is not stored in CAD (NDF) files and they must be chosen carefully by the end users. In practice, the number of points required is more than hundreds per board, i.e. setting up a PCB during test development can be very time consuming and painstaking.
Furthermore, in any optical metrology that involves moiré and phase shift profilometry techniques are usually faster but they are normally used for measurements that are related to surface deformations. The surface deformations include height measured with respect to its neighboring objects in a continuous manner, for e.g. a human face or components on board and not absolute height measurement.
There remains a need in the art for a method to improve system call rate and test coverage for the automated x-ray inspection (AXI) machine. Henceforth, there exists a need in the art for a method and system to reduce false call and false rejects in automated x-ray inspection (AXI) machine.